You have two directories because one is to contain the files Apache needs to execute directly to serve the pages. The other directory has a few critical files...config.php for instance...that you do not want outsiders to access via Apache, or your site will be hacked. The php scripts under public_html can execute the stuff in this other directory, but outsiders can't directly see it.
The directories you're asking about, backend, images, etc, all do live under the public_html directory. Anything you uncompress that shows up under public_html needs to stay there. And public_html might not be named that on your server, it might be www, htdocs, or some other name.
See the following link,, it's really critical for you to understand what the directories look like. Plan on reviewing this for a few hours while you set up your site. Graphical Instructions